Seven content categories

Metadata content is content that sets up the presentation or behavior of the rest of the content, or that sets up the relationship of the document with other documents, or that conveys other “out of band” information.

base, command, link, meta, noscript, script, style, title

Flow content

Most elements that are used in the body of documents and applications are categorized as flow content.

Palpable content again

We would generally expect text inside a p element, but it’s easy to imagine legitimate cases in which the paragraph is empty.

This reflects a general rule that is not a hard requirement: elements whose content model allows any flow or phrasing content should have at least one child node that is palpable, and that does not have the hidden attribute specified.

Transparency

Some elements have a transparent content model. They inherit their content model from their parent. eg the content model for a elements:

Transparent, but there must be no interactive content descendant.

If the element has no parent then it accepts flow content. This case is hard to imagine, since body accepts flow content and head accepts metadata content.